Dispersion managed optical waveguide fiber

ABSTRACT

Disclosed is a single mode optical waveguide fiber designated for compensating for positive dispersion in optical telecommunications systems. A key characteristic of the invention is that the novel dispersion compensating waveguide, viz., a waveguide having large negative dispersion, contains no dopants, such as fluorine, which lower the refractive index of silica. A refractive index profile design which includes a high refractive index center segment ( 5 ) surrounded by a plurality of alternating high ( 6, 10, 13 ) and low refractive index segments, provides a dispersion compensation fiber which has the optical properties required for the system to be compensated without sacrificing bend resistance, increasing splicing loss, or elevating polarization mode dispersion.

This application is a 371 of PCT/US98/15274 filed Jul. 23, 1998 whichclaims benefit of 60/054,821 filed Aug. 7, 1997.

BACKGROUND OF THE INVENTION

The invention is directed to an optical waveguide fiber having amulti-ring core design which provides for large negative dispersion. Inparticular, the large negative dispersion is achieved while maintaininglow bend loss, low splice loss, and polarization mode dispersioncomparable to that of the waveguide fiber comprising the link to becompensated.

Many telecommunications links designed to operate in a wavelength windownear 1300 nm have been installed. In general the waveguides manufacturedfor such links were designed to have a zero dispersion wavelength near1300 nm to avoid signal distortion due to dispersion over long,unregenerated link lengths. More recently, the capability of operatingat a wavelength window near 1550 nm has been developed. The 1550 nmwindow is advantageous because the attenuation of a silica basedwaveguide has a minimum there and the window lies near the center of theerbium doped optical amplifier gain curve. In fact, for a typicalwaveguide the attenuation near 1550 nm is less than 60% of theattenuation near 1300 nm. This large gain in signal to noise ratio aswell as the possibility of extending link length without addingregenerators has made telecommunications operations at 1550 nm veryattractive.

However, if a telecommunication link originally made for 1300 nm windowoperation is to be upgraded to include 1550 nm operation, the dispersionpenalty due to the location of the dispersion zero must be overcome.Because the waveguides in these links weredesigned to have zerodispersion near 1300 nm, the dispersion at 1550 nm increases rapidlywith link length. The dispersion at 1550 nm is typically about 15-20ps/nm-km.

Two strategies which may be used to remove the 1550 nm dispersionpenalty are:

employ very narrow line width 1550 nm lasers; or,

introduce dispersion compensating waveguide fiber into the link.

A dispersion compensating waveguide fiber is one which has a dispersionof opposite sign relative to the link dispersion which is to becompensated. For example, a 1300 nm telecommunications system may have18 ps/nm-km dispersion at 1550 nm. A link length of 60 km is common.Thus, for 1550 nm operation over this link 1080 ps per nm of laser linewidth must be compensated to avoid a dispersion penalty. Even thoughvery narrow line width distributed feedback lasers have been developed aconsiderable amount of dispersion remains to compensated at operatingwavelengths far from the zero dispersion wavelength. Thus the dispersioncompensating waveguide fiber can be used to advantage in systemsemploying very narrow linewidth lasers as well as lasers having arelatively broad emission band.

The technical innovations required to implement either of thesestrategies are sophisticated. In the case of the compensation waveguidestrategy, waveguide core profiles which provide the proper amount ofcompensating dispersion must be found. The problem is complicated by thefact that changing the core refractive index profile to obtain anegative, i.e., compensating, dispersion, changes other properties ofthe waveguide. In particular, waveguide fibers having negativedispersion have been found to be more susceptible to bending loss, havehigher polarization mode dispersion, and increased attenuation whencompared to the original waveguide fiber used in the system. See. forexample U.S. Pat. No. 5,361,319, Antos, et al.

The telecommunications industry, therefore, has a need for a dispersioncompensating waveguide fiber which:

is resistant to bending loss;

has a high negative dispersion so that the compensating fiber length isrelatively short,

has a low attenuation;

exhibits low splice loss with the original system waveguide; and,

has comparatively low polarization mode dispersion.

DEFINITIONS

The radius or width of the regions of the core are defined in terms ofthe index of refraction of the core along a radius. A particular regionbegins at the point where the refractive index characteristic of thatregion begins and ends at the last point where the refractive index ischaracteristic of that region. Radius and width will be expressed interms of these beginning and ending points unless otherwise noted in thetext.

An alpha refractive index profile is n=n₀(1−Δ(r/a)^(α)), where n₀ is therefractive index at the first point of the alpha index profile, Δ isdefined below, r is radius, and a is the radius measured from the firstto the last point of the alpha index profile, and r is chosen to be zeroat the first point of the alpha index profile.

The width of an index profile segment is the distance between twovertical lines drawn from the respective beginning and ending points ofthe index profile segment to the horizontal axis of the chart ofrefractive index vs. radius.

The % index delta is % Δ=[(n₁ ²−n_(c) ²)/2n₁ ²]×100, where n₁ is a coreindex and n_(c) is the clad index. Unless otherwise stated, n₁ is themaximum refractive index in the core region characterized by a % Δ.

The zero reference for refractive index is chosen as the minimumrefractive index in the clad glass layer. A region of refractive indexin the core which is less than this minimum value is assigned a negativevalue.

Bend performance is defined by a standard testing procedure in which theattenuation induced by winding a waveguide fiber about a mandrel ismeasured. The standard test calls for waveguide fiber performance withone turn about a 32 mm mandrel and with 100 turns about a 75 mm mandrel.The maximum allowed bending induced attenuation is usually specified inthe operating window around 1300 nm and around 1550 nm.

SUMMARY OF THE INVENTION

The invention set forth in this application meets the need for a highperformance dispersion compensating optical waveguide fiber withoutusing fluorine.

A first aspect of the invention is a dispersion compensating single modeoptical waveguide fiber having a central core region surrounded by aclad layer. To make the structure a waveguide, at least a portion of thecore refractive index must be higher than the maximum refractive indexof the clad layer which abuts and surrounds the core region. The coreregion of the novel dispersion compensating waveguide comprises at leastfive segments, a center segment symmetric about the long axis of thewaveguide, and at least four annular segments symmetrically layeredabout the center segment. The center segment has a relative index,Δ_(c)%, which is in the range of 1.5% to 3.5%. The upper limit on Δ_(c)%depends upon what is practical in terms of doping capability and interms of added attenuation as dopant percent increases. Large negativedispersion can be obtained using a center relative index higher than3.5%, but dopant levels high enough to produce such an index are usuallyimpractical and not cost effective. A preferred range for Δ_(c)% is 2%to 3%.

The magnitudes of the relative indexes are all positive and are chosenas follows: the center relative index is largest; the successive oddnumbered surrounding segments or layers, beginning with the number 1 forthe layer abutting the center segment, are smaller in magnitude thanΔ_(c)%; the successive even numbered surrounding layers are smaller inmagnitude than Δ_(c)% but larger in magnitude than the odd numberedlayers.

The center segment is characterized by a radius and the successiveannular segments are characterized by widths. The novelty of thewaveguide structure is defined by the choice of relatives indexes, Δ%,and the center radius and widths of the annular segments which make upthe core. In particular, the Δ%'s and radius and widths are chosen suchthat the total dispersion of the waveguide is more negative than about−85 ps/nm-km. The waveguide attenuation depends upon the relative indexof the center segment, Δ_(c)%. For Δ_(c)% near 2% the attenuation isless than about 0.55 dB/km over a pre-selected band of wavelengths whichlies in the wavelength range 1520 nm to 1600 nm. A wavelength of choiceis near 1550 nm, which is an optimum operating wavelength fortelecommunications. Average attenuation of 0.46 to 0.48 dB/km is typicalfor waveguide profiles having Δ_(c)% near 2%. As Δ_(c)% approaches 3% to3.5%, the upper limit of attenuation is about 1.5 dB/km. However, thehigher Δ_(c)% profiles yield a larger negative dispersion, which means ashorter length of compensating waveguide is required. The noveldispersion compensating waveguide may be tailored for uses in which alonger length, lower attenuation waveguide is preferred as well as foruses in which a shorter length of waveguide is preferred, therebyrequiring the waveguide to have a larger negative dispersion.

Further, the novel waveguide is characterized by an average splice lossof 0.5 dB and a bend loss no greater than 0.025 dB. The splice lossrefers to splicing between the compensating waveguide and a waveguide inthe system being compensated, which is usually a step index waveguidewhich is a standard in the industry. The bend resistance refers to themandrel tests described in the definitions section.

In addition, the polarization mode dispersion is substantially the sameas that for the waveguides of the system being compensated. Thus, thepolarization mode dispersion may be expected to be no greater than about0.5 ps/km^(½).

In any of the embodiments described above, the shape of the respectivecore segments can be chosen from among the group of α-profiles, roundedstep profiles, or trapezoidal profiles. The preferred shape is anα-profile with α=0 to 6 or a rounded step. Of these, the α=1 to 2 is amost preferred profile.

The Δ%'s of the even numbered annular segments or layers lie within therange of 0.2 to 0.6 of the center relative index, Δ_(c)%. As is notedabove, the odd numbered segments are lower in magnitude than the evennumbered segments. A particular embodiment of the invention has oddnumbered segments for which the relative indexes are flat and close tozero.

A detailed embodiment of the 5 segment core (4 annular segments), inwhich the overall core radius is in the range 12 μm to 18 μm, is givenin an example below.

Another embodiment described in detail below is a 7 segment core, i.e.,one having 6 annular segments, in which the overall core radius is inthe range 15.5 μm to 23.5 μm.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an illustration of the novel index profile showing the centersegment and the annular core layers.

FIG. 2 is a measurement of a waveguide profile in which certain of thecore segments have an index less than the clad index.

FIG. 3 is an index measurement of a 3 segment profile.

FIG. 4 is an index measurement of a 5 segment profile.

FIG. 5 is an index measurement of a 7 segment profile.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The novel refractive index profile described herein achieves the goal oflarge negative dispersion while retaining the advantageous properties ofrelatively low attenuation, low splice loss and bend loss, andpolarization mode dispersion comparable to that of standard single modeoptical waveguide fiber. In addition, the large negative dispersion isachieved without recourse to the use of dopants, such as fluorine, whichreduce refractive index to less than that of silica. Use of fluorine canadd several steps to the waveguide manufacturing process. Also, theplacement is such that dopant type or concentration interfaces occur inthe light carrying region of the waveguide. Thus a process whicheliminates fluorine greatly facilitates and cost reduces themanufacturing process.

The large negative dispersion provides for dispersion compensation usinga relatively short length of compensating waveguide fiber, thus limitingthe additional attenuation introduced into the system being compensated.

A general profile illustrative of the novel refractive index is shown inFIG. 1 which is a chart of relative index vs. waveguide radius. The mainfeatures of the refractive index profile are the high index centersegment 2, the abutting annulus 4 of lower index, and a plurality ofhigher index rings exemplified by rings 6, 10, and 13. Alternativeshapes for the respective center segment, low index segment and higherindex segment are shown as dashed curves 5, 9, and 7. The definition ofthe width of an annular region is shown by horizontal axis segment 12.The end points defining the width may be conveniently chosen as thepoints at which the refractive index slope changes rapidly or changesfrom a constant as in the detailed embodiments given below. Note thatthe annular width is measured along the horizontal axis, i.e., the Δ=0line. Another major feature of the refractive index profile is that Δ%is nowhere negative, wherein silica is the reference refractive index.Thus the advantageous characteristics of the novel waveguide areachieved without resorting to index lowering dopants which typically aremore difficult to incorporate and control. That is, undesirable multipleprocess steps and the introduction of an interface in the light carryingregion of the waveguide are advantageously avoided.

A measured refractive index profile of a prior art dispersioncompensating waveguide is shown in FIG. 2. Two regions of the core indexprofile, the center segment 14 and the ring segment 18 have positive Δ%.Both of the lower index segments, annular segments 16 and 20, have aminimum index which is negative, meaning that the two segments containan index towering dopant. A negative dispersion more negative than −70ps/nm-km is possible using this profile design. However, in part due tothe width and positioning of annular segment 18, the bend and splicingperformance and the polarization mode dispersion of the waveguide arenot as good as standard single mode fiber. This performance deficiencytogether with the requirement of negative Δ%'s prompted theinvestigation of alternative profiles.

A measurement of a three segment profile is shown in FIG. 3. The centersegment 22 is abutted by a low index annular segment 23 which in turn isabutted by a higher index annular ring 24. The design is simpler thanthat of FIG. 2 and the narrowing of annulus 24 as compared to annulus 18of FIG. 2, provides for improved bend and splicing performance. However,the dispersion was found to be about −65 ps/nm-km which increases thelength of the dispersion compensating fiber and so introduces a greaterattenuation into the dispersion compensated system.

By comparison the novel refractive index profile shown measured in FIG.4 provides for a dispersion more negative than about −85 ps/nm-km. Thecenter segment, 26, has a Δ% is in the range of 1.5 to 3.5%.

In an embodiment in which Δ_(c)% is 2%, the radius 28 of the centralsegment is in the range 2 μm to 3 μm. The remainder of the corecomprises four annular segments, 30, 32, 34, and 36, surrounding thecenter segment. The respective relative indexes follow the rule,Δ_(c)%>Δ₂%≧Δ₄%>Δ₁%≧Δ₃%≧0. The widths of the, for this case in whichΔ_(c)% is 2%, respective annular segments, w₁, w₂, w₃, and w₄ are in theranges 2.4 μm to 3.6 μm, 1.6 μm to 2.4 μm, 0.8 μm to 1.2 μm, and 1.6 μmto 2.4 μm. In this case the widths are taken as the points at thebeginning and end of a segment at which the slope of the index profilechanges from a constant value. This definition of width is illustratedin FIG. 4 as lines 38 and 40. The total core radius 42, which is the sumof the central radius, the segment widths, and the index down taper tothe clad layer index is 12 μm to 18 μm.

As Δ_(c)% tends toward its upper limit of about 3.5%, the radius of thecentral region is reduced, For example at Δ_(c)%=3%, the radius of thecentral segment is in the range 1.2 μm to 1.8 μm. The widths of theremaining segments are not changed appreciably. The Δ%'s of the annuli32 and 36 lie in the range of 0.2 to 0.6 of center relative indexΔ_(c)%. The Δ% of the low refractive index annular segments 30 and 34are typically less than 20% of the higher index annular ring segmentsand may be advantageously chosen to be at or near 0.

The novel refractive index profile illustrated in FIG. 4 has a highnegative dispersion as well as good bend resistance and low splice loss.In addition, the other optical properties of the novel waveguide aresuch that it is suitable for use as a dispersion compensating waveguide.The high negative dispersion allows for compensation of positivedispersion in a telecommunications link using a relatively short lengthof the compensating waveguide. The relatively low attenuation, which canbe less than 0.5 dB/km, depending upon choice of Δ_(c)%, of thecompensating length of waveguide allows for acceptable signal to noiseratio in the system without need for an additional signal regenerator.

The seven segment embodiment of the novel refractive index isillustrated in FIG. 5. In the case in which Δ_(c)% is about 2%, thecentral segment 46 has radius 44 in the range 2 μm to 3 μm and therespective widths w₁, w₂, w₃, w₄, w₅, and w₆, which are shown as 48, 50,52, 54, 56, and 58 in FIG. 5., are in the ranges 2.9 μm to 4.4 μm, 1.25μm to 1.90 μm, 0.75 μm to 1.10 μm, 0.9 μm to 1.35 μm, 0.9 μm to 1.35 μm,and 1.65 μm to 1.10 μm. These widths are defined as stated above in the5 segment embodiment. The total core radius 60, which is the sum of thecentral radius, the segment widths, and the index down taper to the cladlayer index is 15.70 μm to 23.50 μm.

The relative indexes of the core having seven segments, 46, 62, 64, 66,68, 70 and 72, follow the rule, Δ_(c)%>Δ₂%≧Δ₄%≧Δ₆>Δ₁≧Δ₃%≧Δ₅%≧0. AsΔ_(c)% moves toward its upper limit of 3.5%, the central segment radius44 decreases. For example, at Δ_(c)%=3%, the radius of the centralsegment is in the range 1.2 μm to 1.8 μm. The widths of the remainingsegments are not changed appreciably. as stated for the embodiment ofFIG. 4 described immediately above. The three high index annularsegments 64, 68, and 72 are in the range of 0.2 to 0.6 of the centerrelative index Δ_(c)%. The Δ% of the low refractive index annularsegments 62, 66, and 68 are typically less than 20% of the higher indexannular ring segments and may be advantageously chosen to be at or near0.

The formation of a very high Δ% center together with a set of high Δ%annular segments spaced apart from the center segment and from eachother by low index segments provides the surprising characteristic oflarge negative dispersion combined with excellent confinement of thesignal light as shown by the good bend resistance.

Additional unusual and advantageous results are:

attenuation is low;

cut off wavelength is compatible with the original system;

splice loss is low; and,

polarization mode dispersion is not degraded relative to the originalsystem.

Although particular embodiments of the invention have been disclosed anddescribed herein, the invention is nonetheless limited only by thefollowing claims.

I claim:
 1. A dispersion compensating single mode optical waveguidefiber comprising: a central core glass region, surrounded by and incontact with a clad glass layer, at least a portion of the core glassregion having a refractive index higher than the maximum refractiveindex of the clad layer, the central core glass region having a center,a refractive index profile, and a radius, and the clad glass layerhaving a refractive index profile, in which, the refractive indexprofile of the central core region includes at least five segments, acenter segment having a cross sectional area distributed substantiallysymmetrically about the core center and having a relative index Δ_(c)%,which is in the range of 1.5% to 3.5%, and a radius, a number j ofannular segments surrounding the center segment beginning with a firstannular segment abutting the center segment, a second annular segmentabutting the first annular segment and a jth annular segment abuttingthe (j−1)th segment to form a core having a center segment and j annularsegments, the respective annular segments having a relative index Δ_(j)%and a width, w_(j), measured along the Δ%=0 line, where j is an integer≧4; in which Δ_(c)%>Δ_(j)%≧0 for all values of j, and Δ_(j) for j aneven number is greater than Δ_(j) for j an odd number; in which therespective relative indexes, the center segment radius and annularsegment widths are selected to provide a dispersion more negative thanabout −85 ps/nm-km at a pre-selected wavelength.
 2. The dispersioncompensating single mode waveguide of claim 1 in which the attenuationis no greater than about 1.5 dB/km, and for Δ_(c)% near 2% is no greaterthan 0.55 dB/km, over a pre-selected band of wavelengths containedwithin the range 1520 nm to 1600 nm, and a splice loss which has anaverage of 0.5 dB for splices between the dispersion compensatingwaveguide and standard step index single mode telecommunicationwaveguide fiber.
 3. The dispersion compensating single mode waveguide ofclaim 1 in which j=4 and the respective relative indexes are such thatΔ_(c)%>Δ₂%≧Δ₄%>Δ₁%≧Δ₃%≧0 and the core radius is in the range 12 μm to 18μm.
 4. The dispersion compensating single mode waveguide of claim 1 inwhich j=6 and the respective relative indexes are such thatΔ_(c)%>Δ₂%≧Δ₄% ≧Δ₆>Δ₁≧Δ₃%≧Δ₅%≧0 and the core radius is in the range 15.5μm to 23.5 μm.
 5. The dispersion compensating single mode waveguide ofclaim 3 or 4 in which the segments comprising the core each have ashape, the shape of the center segment selected from the groupconsisting of an α profile, a rounded step index profile, and atrapezoidal profile.
 6. The dispersion compensating single modewaveguide of claim 5 in which α is in the range of about 0 to
 6. 7. Thedispersion compensating single mode waveguide of claim 3 in which theprofile shape of annular segments 1 and 3 is a horizontal line andΔ₁%=Δ₃% and Δ₁% is substantially zero and Δ₂% and Δ₄% may individuallytake on values in the range of 0.2 to 0.6 times the value of Δ_(c)%. 8.The dispersion compensating single mode waveguide of claim 4 in whichthe profile shape of annular segments 1, 3, and 5 is a horizontal lineand Δ₁%=Δ₃%=Δ₅% and Δ₁% is substantially zero and Δ₂%, Δ₄% and Δ₆% mayindividually take on values in the range of 0.2 to 0.6 times the valueof Δ_(c)%.
 9. The dispersion compensating single mode waveguide of claim3 in which the profile shape of annular segments 2 and 4 is selectedfrom the group consisting of an α profile, a rounded step index profile,and a trapezoidal profile.
 10. The dispersion compensating single modewaveguide of claim 4 in which the profile shape of annular segments 2,4, and 6 is selected from the group consisting of an α profile, arounded step index profile, and a trapezoidal profile.
 11. Thedispersion compensating single mode waveguide of claim 7 in which therespective widths of the annular regions w₁, w₂, w₃, and w₄ are in theranges 2.4 μm to 3.6 μm, 1.6 μm to 2.4 μm, 0.8 μm to 1.2 μm, and 1.6 μmto 2.4 μm.
 12. The dispersion compensating single mode waveguide ofclaim 8 in which the respective widths w₁, w₂, w₃, w₄, w₅, and w₆ are inthe ranges 2.9 μm to 4.4 μm, 1.25 μm to 1.90 μm, 0.75 μm to 1.10 μm, 0.9μm to 1.35 μm, 0.9 μm to 1.35 μm, and 1.65 μm to 1.10 μm.